The authors report on a tunable and movable liquid microlens in situ fabricated through fluid manipulation within microfluidics. Taking advantage of surface tensions at the microscale, the microlens is formed by a liquid droplet interfacing air. Through pneumatic control of the droplet, the microlens can be tuned in focal length and moved within the microfluidic channel on demand, thus being highly reconfigurable. A focal length tuning range from 1.5  to  8.9  mm is demonstrated. The in-plane optical axis of the microlens provides the flexibility in designing micro-optics within microfluidics, as demonstrated by realizing a planar fluorescence detection device.